High impact foam blend of two different polyethylenes



United States Patent Office US. Cl. 260-25 6 Claims ABSTRACT OF THE DISCLOSURE Foamed polymeric compositions having high impact resistance are produced by foaming a blend of relatively low density, broad molecular weight distribution polyethylene, relatively high density, narrow molecular weight distribution polyethylene and a selected amount of foaming agent.

Several properties of foamable thermoplastic compositions are of considerable importance in many of their applications. For example, it is often desirable that foamed thermoplastic articles possess relatively uniform cell structure as well as sufiicient tensile strength, resilience, implication. We have found that several desirable properties of such foamable compositions and foamed article produced therefrom can be improved by employing a particular thermoplastic composition and selected amounts of several polymers and foaming agent.

In the past, various blends of polyethylene have been formulated to produce compositions having improved physical charactistics. For example, a polyethylene blend having improved properties is prepared from low density polyethylene and cracked highly crystalline high density polyethylene as disclosed in US. Patent 3,086,- 95 8. Low density polyethylene has been blended with visbroken high density polyethylene to produce compositions having improved processability and permeability as disclosed in US. Patent 3,247,290. Stress cracking properties of high molecular weight high density polyethylene are improved by blending them with a minor amount of low molecular weight low density polyethylene as disclosed in US. Patent 3,183,283.

We have now discovered that foamed polymer compositions having greatly improved combination of desired properties can be produced from blends of low density polyethylene as produced, particle form high density polyethylene as produced and a blowing agent. Particle form high density polyethylenes are made according to U. S. Patent No. 2,825,721 and British Patent No. 853,- 414.

It is therefore one object of this inevntion to provide a foamed structurally stable thermoplastic composition having improved physical properties. It is another object of this invention to provide foamed compositions having uniform cell structure and improved impact resistance. It is yet another object of this invention to provide an improved foamable composition for producing structurallv stable foamed thermoplastic articles having uniform cell structure and improved physical properties.

In accordance with one embodiment of this invention, foamed polyethylene articles having improved physical properties are pro-duced from foamable compositions comprising blends of low density polyethylene, high density polyethylene, high density polyethylene and a foaming agent.

The low density polymers usually have densities within the range from about 0.89 to about 0.93. However, polymers having densities of about 0.91 to about 0.93 are presently preferred. Within the concept of this invention, these polymers can be employed in the foamable composi- 3,496,124 Patented Feb. 17, 1970 tions as produced in which case they usually possess melt index values within the range of from about 11 to about 13. The molecular weight distributions of these low density polymers are usually relatively broad as represented by MI/CIL ratios in excess of 1.2: 1. These ratios are as determined by ASTM Standard B1238-62T melt index analysis and Canadian Industries Limited Standard (CIL) values.

Although essentially any polyethylene polymers having the described properties can be used effectively within the concept of this invention, it is presently preferred to employ the low density polymers produced by the well known high pressure polymerization process without substantial physical modification.

The high density polyethylenes usually have densities within the range of from about 0.94 to about 0.97 and melt index values within the range of from about 0.1 to about 1.0 as determined by ASTM Standard B1238-62T. The molecular weight distributions of these polymers are usually more narrow than those of the above-described low density polymers as represented by MI/CIL ratios of less than 1.2: 1, preferably within the range of from about 0.8:1 to about 1.2:1.

High density polyethylene having these desired properties can be produced by the process described in US. Patent No. 2,825,721, although polymers having the desired properties yet produced by other processes can also be employed within the concept of this invention.

Foaming agents which can be employed in this invention are essentially any of those which can be activated within the operating temperatures desired during the molding step. Although molding temperatures can vary consid erably, depending upon the polymers employed, the preferred operating ranges associated with the preferred polymers is usually from about 325 to about 500 F. For example, carbonates and bicarbonates of sodium, potassium, ammonium, oxalates of sodium and calcium, sodium formate, diazo compounds such as amino-diazo-benzene, azo bisformamide, azobisisobutyronitrile and numerous others are adequate for this purpose. At present, azobisformamide, marketed as Celogen AZ by Naugatuck Chemical Corp., is the preferred blowing agent.

In some applications it is desirable that a dispersing agent also be used. Numerous dispersants can be employed, as their particular characteristics are not necessarily essential to the concept of this invention. Suitable dispersants are, for example, relatively low molecular weight polyisobutylene, mineral oil, dibutyl phthalate, polyethylene glycols, butyl stearate and numerous others well known in the art. Polyisobutylene haveing a molecular weight within the range of from about 600 to about 1200 is presently preferred and can be employed in concentrations within the range of from about 0.05 to about 0.1 weight percent based on polymer.

Numerous fillers, pigments, lubricants, antioxidants, etc. well known in the art can also be incorporated into these compositions as desired.

The foamed compositions of this invention having improved physical properties are prepared from blends comprising from about 65 to about parts by weight low density polyethylene and from about 35 to about 20 parts by weight high density polyethylene. These polymers can be blended by any means sufficient to produce a relatively homogeneous blend such as, for example, blending in a drum blender or on a Banbury Mixer or extruder blending and pelletizing. Foaming agent concentration is de termined by the degree of foaming desired and the necessary characteristics of the resultant foam structure required to give the improved impact resistance. Foaming agent concentrations within the range of from about 0.75 to about 1.1 achieve these purposes and are presently preferred.

The concept of this invention is illustrated by the following examples.

Several blends having varying amounts of high density (.954) and low density (.915) polyethylenes and Celogen AZ (asobisformamide) were blended in a Rainco 4 of this latter blend is also substantially below the desired leveis.

We claim: 1. A foamable injection moldable composition for producing structurally stable foamed articles having an im- Model 101 drum blender operated at 50 r.p.m.s and at 5 proved property profile comprising a blend of from about ambient temperature for a period of minutes to pro 65 to about 80 parts by weight of a relatively low density duce homogeneous blends thereof and were injection broad molecular weight distribution polyethylene, from molded into foamed buckets. The impact resistance, about 35 to about 20 parts of relatively high density fiexural modulus and tensile strength (MD and TD) of 10 narrow molecular weight distribution polyethylene, and the foamed articles were determined by the methods de from about 0.75 to about 1.1 weight percent foaming fined by ASTM Standards D790-63, D638-61T, agent based on said polyethylene, said low density polyand D256-56, respectively. Rim stiffness values were deethylene having a density within the range of 0.89-0.93 termined with a standard Instron machine and a 10%6 and an MI/CIL (molecular weight distribution) ratio inch rim diameter, 9% inch high bucket having a wall in excess of 12:1, and said high density polyethylene thickness of 160 mils. Rim stiffness is reported as the having a density within the range 0.94-0.97, and an pounds force required to compress the rim one inch at MI/CIL (molecular weight distribution) ratio less than a ratio of two inches per minute. The results are illustrat- 1.2:1. ed in the following table. 2. The composition of claim 1 wherein said low den- Rim Blend Blowing stifi- Tensile strength ratio, agent, Impact, ness, Flexural HD/LDPE percent 1t./'lbs. lbs. modulus MDI TD 1 0 100 1. 00 83 1.5 15,000 025 540 25 75 None as 4.9 41,000 1,000 1,500 50 0. 75 50 3.5 34,000 050 1,050 75 0. 75 42 4.1 42,000 1,100 1,150 25 75 1. 00 24.4 4.0 33, 000 1,100 812 70 1. 00 27.2 5.0 45, 000 1,350 1,100 55 1.00 15 5.4 51,000 1,400 1,250 1.25 5.3 6.2 50,000 1,500 1,300

1 MD, Machine direction.

2 TD, Transverse direction.

From these data, it is apparent that concentrations of 35 sity polyethylene has a density within the range of from either foaming agent or high density polyethylene above about 0.91 to about 0.93 and a melt index within the the prescribed levels result in considerable reduction of range ll13, and said high density polyethylene has a impact resistance, lower tensile strengths and overall unmelt index within the range 0.11.0. desirable property profile. Blend No. 1 of foamed low 3. The composition of claim 1 wherein said low dendensity polyethylene resulted in a soft semi-resilient arti- 40 sity polyethylene has an MI/CIL ratio within the range cle having a rim stiffness of only 1.5 lbs. and a flexural of from about 1.5 to about 25:1 and said high density modulus of only 15,000. Although this material might polyethylene has an MI/CIL ratio within the range of be suitable for the production of sponge or articles havf a out 0.8 to about 1.211 ing similar characteristics, it is not considered suitable, T 90111130513011 of claim 1 Wherelll 9 foamlng due to the nature of the structural qualities mentioned, agent 13 decomposable at a temlljefatul'e Wlthlfl the range for application in the production of buckets, coat hangof from about 325 3 about 475 ers, glasses, etc. require considerably higher structural foamefl artlcle Pmduced by heatmg the stability. Blend No. 2 of unfoamed polyethylene constil g Claim 1 at a temperature sufficmnt to decom' pose sat oammg agent. tuted F l propomons of hlgh and low dlenslty poly 50 6. A foamed injection molded article produced by mer within the preferred range for the foamed blends, 1 d d f m d d bl heating the composition of claim 3 at a temperature suitilc es Pro um an eilce a a e cient to decompose said foaming agent. property profile but are quite heavy having densities of 0.922 (gram per 00.). Blends 3, 4, 5 and 6 are the most References Cited desirable of those illustrated in the table in that they not only possess the desired structural qualities but have UNITED STATES PATENTS considerably reduced density of 0.645 gram per cc. 3,093,331 7/ 1963 0- Blend 7 is considered to be of marginal value due to the resulting decrease in impact strength and Blend No. MURRAY TILLMAN Pn'mary Exammer 8 is considered unsuitable in these applications due to MORTON FOELAK, Assistant EXaIIliIlBr the lack of resilience and the relatively high brittleness found in these articles as evidenced by high flexural modulus and high rim stiffness. The impact resistance US. (:1. X.R. 

